The present disclosure relates to devices and methods that permit fixation and stabilization of the bony elements of the skeleton. The devices permit adjustment and maintenance of the spatial relationship(s) between neighboring bones. Depending on the specifics of the embodiment design, the motion between adjacent skeletal segments may be maintained, limited or completely eliminated.
Spinal degeneration is an unavoidable consequence of aging and the disability produced by the aging spine has emerged as a major health problem in the industrialized world. Alterations in the anatomical alignment and physiologic motion that normally exists between adjacent spinal vertebrae can cause significant pain, deformity, weakness, and catastrophic neurological dysfunction.
Surgical decompression of the neural tissues and immobilization of the vertebral bones is a common option for the treatment of spinal disease. In addition to mechanical fixation, a bone graft or comparable bone-forming material is used to connect the vertebral bones and, with ossification of the graft material, the vertebral bodies are fused together by the bony bridge. Currently, mechanical fixation is most frequently accomplished by anchoring bone screws into the pedicle portion of each vertebral body and then connecting the various screw fasteners with an interconnecting rod. The screw/rod construct produces rigid fixation of the attached bones.
The growing experience with spinal fusion has shed light on the long-term consequences of vertebral immobilization. It is now accepted that fusion of a specific spinal level will increase the load on, and the rate of degeneration of, the spinal segments immediately above and below the fused level. As the number of spinal fusion operations have increased, so have the number of patients who require extension of their fusion to the adjacent, degenerating levels. The rigidity of the spinal fixation method has been shown to correlate with the rate of the degenerative progression of the adjacent segments. In specific, implantation of stiffer instrumentation, such as rod/screw implants, produced a more rapid progression of the degeneration disease at the adjacent segment than use of a less stiff fixation implant.
An additional shortcoming of the traditional rod/screw implant is the large surgical dissection required to provide adequate exposure for instrumentation placement. The size of the dissection site produces unintended damage to the muscle layers and otherwise healthy tissues that surround the diseased spine. A less invasive spinal fixation implant would advantageously minimize the damage produced by the surgical exposure of the spine.
Fixation of the spinous process segment of adjacent vertebrae provides a less rigid and less invasive method of vertebral fixation. Kapp et al. in U.S. Pat. No. 4,554,914 issued Nov. 26, 1985 disclosed a device of two elongated plates that are adapted to clamp onto adjacent spinous process. The plates are disadvantageously connected by locking bolts that transverse the substances of each spinous process. Bolts placed in this configuration will necessarily weaken the bony elements and lead to spinous process fractures and construct failure. Howland et al in U.S. Pat. No. 5,496,318, issued Mar. 5, 1996 disclosed the placement of an inter-spinous process spacer and encircling tension band to reduce vertebral motion. While the device can reduce vertebral flexion and extension, it can not effectively resist vertebral movement in the other motion planes. In U.S. Pat. No. 6,312,431 issued Nov. 6, 2001, Asfora disclosed a device comprised of two opposing plates that are interconnected by a malleable tether and adapted to capture the adjacent spinous processes between them. As with the Howland device, the fixation strength of this implant is limited by the mobile interconnecting tether. As such, neither implant can effectively immobilize the vertebral bones in all relevant motion planes. The lack of fixation significantly increases the possibility that the bone graft will not heal, the vertebral bones will not fuse, the construct will fail and the patient will develop chronic pain.
Superior immobilization devices were disclosed by Robinson et al. in U.S. Pat. No. 7,048,736 issued May 23, 2006 and by Chin et al. in U.S. Pub. Nos. 2007/0179500, 2007/0233082 and 2007/0270840. Each of these documents disclosed plates (or segments thereof) that engage each side of two adjacent spinous processes, wherein the plates are interconnected by a rigid member that resides within the interspinous space. Mechanical testing of the Robinson device was recently published by J C Wang et al. in the Journal of Neurosurgery Spine (2006 February; 4(2): 160-4) and the text is hereby incorporated by reference in its entirety. The device was found to be weaker than conventional fixation techniques in all modes of vertebral movement and particularly lacking in fixation of rotational motion. Because of its limited stabilization properties, the device should be used in conjunction with additional implants. (See Wang J C et al. in the Journal of Neurosurgery Spine. 2006 February; 4(2): 132-6. The text is hereby incorporated by reference in its entirety.]
As an additional shortcoming, the Robinson device can not be used to fixate the L5 vertebral bone to the sacrum. The spinous process of the first sacral vertebra is simply too small to permit adequate bone purchase and fixation with either the Robinson or Chin device. Since the L5/S1 level is a frequent site of spinal disease, the inapplicability of these devices at this level is a significant limitation of these implants.
In U.S. Pub. Nos. 2006/0036246, Carl and Sachs disclose a fixation device adapted to fixate the spinous process of one vertebral level to bone screws anchored into the pedicle portion of an adjacent vertebral level. While this invention would permit application at the L5/S1 level and circumvent one disadvantage of the aforementioned spinous process fixation plates, it relies on direct screw fixation into the distal aspect of the spinous process. This technique disadvantageously replicates the inadequate fixation characteristics of the Kapp device previously discussed (U.S. Pat. No. 4,554,914) and carries a high likelihood of spinous process fracture and complete construct failure. Indeed, the inventors try to address this design flaw by augmenting the strength of the spinous process through the use of an internal bone filler or an external brace. Regardless of these efforts, however, the disclosed device provides a cumbersome implant that carries a high likelihood of spinous process fracture and complete loss of vertebral fixation.